Extraction of core samples, or coring, is used in a number of different industries such as the mining industry and the oil and gas industry to obtain information on the quantity and quality of various minerals and hydrocarbon deposits.
Much of the current coring equipment uses a drilling fluid such as mud or water to assist in the cutting of the core sample. For example, a coring barrel may be provided at the end of a drill pipe string and the drill pipe rotated so that the diamond hardened tip of the coring barrel is turned into the formation being cored. However, rotating the coring barrel into the formation may cause glazing damage to the formation. Further, the drilling fluid itself may also damage the formation as well as potentially contaminate the core sample.
It is desirable that as pristine a core sample as possible be obtained from a reservoir with as little damage as possible to the reservoir. This is very difficult in the oil and gas industry as both the drilling fluids and the rotation of the coring barrel into the formation can be damaging to the well formation. Core quality is essential or the information obtained from the core sample can be misleading. Geologists, petrophysicists and reservoir engineers must have accurate lithology, porosity and permeability data from the cores they evaluate.
In addition to obtaining uncontaminated cores, is also desirable to retain the integrity of the core sample as much as possible. This is particularly important when pressure coring, where the pressure on the core sample may decrease when the sample is brought to surface. For example, downhole, the oil and/or water in the formation may contain dissolved gas which is maintained in solution by the downhole pressure. As the pressure on the core sample decreases during the trip to the surface, the dissolved gas may come out of solution and be released.
Thus, critical reservoir fluids such as in-situ gases may be lost or contaminated when using traditional coring methods. Information from these fluids can assist in the core evaluation to determine the most effective drilling, completion, stimulation and extraction methods to use. As well, important economic data such as ultimate resource recovery, capital costs and environmental issues can be more closely defined with better quality core information.
Another major problem that exists with many conventional coring devices and methodologies is that the coring devices can only be used with drill pipe, as the coring devices require the drill string to rotate in order to rotate the core cutting barrel for cutting the core. However, the coring device of the present application does not rely on the rotation of the drill string to operate and, thus, can be used with non-rotating single wall or concentric coiled tubing as well as with conventional jointed drill pipe. Thus, the amount of time and drilling expense in obtaining core samples when using jointed drill pipe, i.e., for tripping drill pipe out of a well one joint at a time, picking up the coring equipment, tripping the drill pipe back in one joint at a time, slowly coring the section of interest, tripping the drill pipe back out one joint at a time, picking up your drilling tools and tripping the drill pipe back in one joint at a time, etc. is greatly reduced. With coiled tubing the time required to trip the tubing in and out of the well is a matter of minutes rather than hours.
The coring device described herein uses air or other gases such as nitrogen to operate the coring apparatus to obtain a core sample, thereby avoiding the problems of existing coring devices with respect to formation damage caused by drilling mud, water or other types of drilling fluids. In particular, low pressure reservoirs can be badly damaged by the hydrostatic weight of drilling fluids when used to cut the core. Furthermore, many formations contain clays that swell once they contact water and can give misleading information on reservoir characteristics if contaminated with drilling muds, drilling fluid and/or water.